Method and apparatus for video coding

ABSTRACT

A method and apparatus for video coding. In some examples, an apparatus includes receiving circuitry and processing circuitry. The processing circuitry decodes prediction information of a block in a current picture from a coded video bitstream. The prediction information includes an index for prediction offset associated with an affine model in an inter prediction mode. The affine model is used to transform between the block and a reference block in a reference picture that has been reconstructed. Further, the processing circuitry determines parameters of the affine model based on the index and a pre-defined mapping of indexes and offset values, and reconstructs at least a sample of the block according to the affine model.

INCORPORATION BY REFERENCE

This present disclosure claims the benefit of priority to U.S.Provisional Application No. 62/698,009, “TECHNIQUES FOR SIMPLIFIEDAFFINE MOTION COMPENSATION WITH PREDICTION OFFSETS” filed on Jul. 13,2018, which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y, or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar motion vector derived from MVs ofneighboring area. That results in the MV found for a given area to besimilar or the same as the MV predicted from the surrounding MVs, andthat in turn can be represented, after entropy coding, in a smallernumber of bits than what would be used if coding the MV directly. Insome cases, MV prediction can be an example of lossless compression of asignal (namely: the MVs) derived from the original signal (namely: thesample stream). In other cases, MV prediction itself can be lossy, forexample because of rounding errors when calculating a predictor fromseveral surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 offers, described here is atechnique henceforth referred to as “spatial merge”.

Referring to FIG. 1, a current block (101) comprises samples that havebeen found by the encoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. Instead of coding that MV directly, the MV can bederived from metadata associated with one or more reference pictures,for example from the most recent (in decoding order) reference picture,using the MV associated with either one of five surrounding samples,denoted A0, A1, and B0, B1, B2 (102 through 106, respectively). InH.265, the MV prediction can use predictors from the same referencepicture that the neighboring block is using.

SUMMARY

Aspects of the disclosure provide method and apparatus for videoencoding/decoding. In some examples, an apparatus includes processingcircuitry for video decoding. The processing circuitry decodesprediction information of a block in a current picture from a codedvideo bitstream. The prediction information includes an index forprediction offset associated with an affine model in an inter predictionmode. The affine model is used to transform between the block and areference block in a reference picture that has been reconstructed.Further, the processing circuitry determines parameters of the affinemodel based on the index and a pre-defined mapping of indexes and offsetvalues, and reconstructs at least a sample of the block according to theaffine model.

In some embodiments, the processing circuitry determines values oftranslational parameters of the affine model according to a motionvector, and determines a non-translational parameter for the affinemodel according to the index and the pre-defined mapping. In an example,the processing circuitry decodes the index for a delta value to adefault of a scaling factor in the affine model, and determines thedelta value according to the index and the pre-defined mapping ofindexes and delta values for the scaling factor. In another example, theprocessing circuitry decodes the index for a delta value to a default ofa rotation angle in the affine model, and determines the delta valueaccording to the index and the pre-defined mapping of indexes and deltavalues for the rotation angle.

In some embodiments, the processing circuitry decodes the index for amotion vector difference, and derives the affine model based on apredicted motion vector and the motion vector difference. In an example,the processing circuitry decodes a first index for a direction of themotion vector difference and a second index for a pixel distance of themotion vector difference. In another example, the processing circuitrydecodes the index for both a direction and a pixel distance of themotion vector difference.

In another example, the processing circuitry decodes indexes for twomotion vector differences respectively for two control points, anddetermines motion vectors respectively for the two control points basedon the two motion vector differences. Then the processing circuitryderives a 4-parameter affine model based on the motion vectors of thetwo control points.

In another example, the processing circuitry decodes indexes for threemotion vector differences respectively for three control points anddetermines motion vectors respectively for the three control pointsbased on the three motion vector differences. Then, the processingcircuitry derives a 6-parameter affine model based on the motion vectorsof the three control points.

In an embodiment, the processing circuitry decodes a first index for afirst motion vector difference for a first control point and predicts asecond motion vector difference for a second control point based on thefirst motion vector difference. Then, the processing circuitry decodes aprediction error to correct the second motion vector difference from thecoded video bitstream. Further, the processing circuitry determines afirst motion vector for the first control point and a second motionvector for the second control point based on the first motion vectordifference and the corrected second motion vector difference, andderives the affine model at least based on the first motion vector forthe first control point and the second motion vector for the secondcontrol point.

Aspects of the disclosure also provide a non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform the method forvideo coding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a schematic illustration of a current block and itssurrounding spatial merge candidates in accordance with H.265.

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system (200) in accordance with an embodiment.

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system (300) in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 5 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 6 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 7 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 8 shows an example of spatial and temporal candidates in someexamples.

FIG. 9 shows examples for UMVE according to an embodiment of thedisclosure.

FIG. 10 shows examples for UMVE according to an embodiment of thedisclosure.

FIG. 11 shows an example of a block with an affine motion model.

FIG. 12 shows examples of affine transformation according to someembodiments of the disclosure.

FIG. 13 shows a flow chart outlining a process example according to someembodiments of the disclosure.

FIG. 14 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 2 illustrates a simplified block diagram of a communication system(200) according to an embodiment of the present disclosure. Thecommunication system (200) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (250). Forexample, the communication system (200) includes a first pair ofterminal devices (210) and (220) interconnected via the network (250).In the FIG. 2 example, the first pair of terminal devices (210) and(220) performs unidirectional transmission of data. For example, theterminal device (210) may code video data (e.g., a stream of videopictures that are captured by the terminal device (210)) fortransmission to the other terminal device (220) via the network (250).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (220) may receive the codedvideo data from the network (250), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (200) includes a secondpair of terminal devices (230) and (240) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (230) and (240)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (230) and (240) via the network (250). Eachterminal device of the terminal devices (230) and (240) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (230) and (240), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 2 example, the terminal devices (210), (220), (230) and(240) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (250) represents any number ofnetworks that convey coded video data among the terminal devices (210),(220), (230) and (240), including for example wireline (wired) and/orwireless communication networks. The communication network (250) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(250) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 3 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (313), that caninclude a video source (301), for example a digital camera, creating forexample a stream of video pictures (302) that are uncompressed. In anexample, the stream of video pictures (302) includes samples that aretaken by the digital camera. The stream of video pictures (302),depicted as a bold line to emphasize a high data volume when compared toencoded video data (304) (or coded video bitstreams), can be processedby an electronic device (320) that includes a video encoder (303)coupled to the video source (301). The video encoder (303) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (304) (or encoded video bitstream (304)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (302), can be stored on a streamingserver (305) for future use. One or more streaming client subsystems,such as client subsystems (306) and (308) in FIG. 3 can access thestreaming server (305) to retrieve copies (307) and (309) of the encodedvideo data (304). A client subsystem (306) can include a video decoder(310), for example, in an electronic device (330). The video decoder(310) decodes the incoming copy (307) of the encoded video data andcreates an outgoing stream of video pictures (311) that can be renderedon a display (312) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (304),(307), and (309) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (320) and (330) can includeother components (not shown). For example, the electronic device (320)can include a video decoder (not shown) and the electronic device (330)can include a video encoder (not shown) as well.

FIG. 4 shows a block diagram of a video decoder (410) according to anembodiment of the present disclosure. The video decoder (410) can beincluded in an electronic device (430). The electronic device (430) caninclude a receiver (431) (e.g., receiving circuitry). The video decoder(410) can be used in the place of the video decoder (310) in the FIG. 3example.

The receiver (431) may receive one or more coded video sequences to bedecoded by the video decoder (410); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (401), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (431) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (431) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (415) may be coupled inbetween the receiver (431) and an entropy decoder/parser (420) (“parser(420)” henceforth). In certain applications, the buffer memory (415) ispart of the video decoder (410). In others, it can be outside of thevideo decoder (410) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (410), forexample to combat network jitter, and in addition another buffer memory(415) inside the video decoder (410), for example to handle playouttiming. When the receiver (431) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (415) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (415) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (410).

The video decoder (410) may include the parser (420) to reconstructsymbols (421) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (410),and potentially information to control a rendering device such as arender device (412) (e.g., a display screen) that is not an integralpart of the electronic device (430) but can be coupled to the electronicdevice (430), as was shown in FIG. 4. The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (420) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (420) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (420) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (420) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (415), so as tocreate symbols (421).

Reconstruction of the symbols (421) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (420). The flow of such subgroup control information between theparser (420) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (410)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (451). Thescaler/inverse transform unit (451) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (421) from the parser (420). The scaler/inversetransform unit (451) can output blocks comprising sample values, thatcan be input into aggregator (455).

In some cases, the output samples of the scaler/inverse transform (451)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (452). In some cases, the intra pictureprediction unit (452) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (458). The currentpicture buffer (458) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(455), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (452) has generated to the outputsample information as provided by the scaler/inverse transform unit(451).

In other cases, the output samples of the scaler/inverse transform unit(451) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (453) canaccess reference picture memory (457) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (421) pertaining to the block, these samples can beadded by the aggregator (455) to the output of the scaler/inversetransform unit (451) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (457) from where themotion compensation prediction unit (453) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (453) in the form of symbols (421) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (457) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (455) can be subject to variousloop filtering techniques in the loop filter unit (456). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (456) as symbols (421) from the parser (420), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (456) can be a sample stream that canbe output to the render device (412) as well as stored in the referencepicture memory (457) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (420)), the current picture buffer (458) can becomea part of the reference picture memory (457), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (410) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (431) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (410) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 5 shows a block diagram of a video encoder (503) according to anembodiment of the present disclosure. The video encoder (503) isincluded in an electronic device (520). The electronic device (520)includes a transmitter (540) (e.g., transmitting circuitry). The videoencoder (503) can be used in the place of the video encoder (303) in theFIG. 3 example.

The video encoder (503) may receive video samples from a video source(501) (that is not part of the electronic device (520) in the FIG. 5example) that may capture video image(s) to be coded by the videoencoder (503). In another example, the video source (501) is a part ofthe electronic device (520).

The video source (501) may provide the source video sequence to be codedby the video encoder (503) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (501) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (501) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (503) may code andcompress the pictures of the source video sequence into a coded videosequence (543) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (550). In some embodiments, the controller(550) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (550) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (550) can be configured to have other suitablefunctions that pertain to the video encoder (503) optimized for acertain system design.

In some embodiments, the video encoder (503) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (530) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (533)embedded in the video encoder (503). The decoder (533) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (534). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (534) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (533) can be the same as of a“remote” decoder, such as the video decoder (410), which has alreadybeen described in detail above in conjunction with FIG. 4. Brieflyreferring also to FIG. 4, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (545) and the parser (420) can be lossless, the entropy decodingparts of the video decoder (410), including the buffer memory (415), andparser (420) may not be fully implemented in the local decoder (533).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (530) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (532) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (533) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (530). Operations of the coding engine (532) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 5), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (533) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (534). In this manner, the video encoder(503) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (535) may perform prediction searches for the codingengine (532). That is, for a new picture to be coded, the predictor(535) may search the reference picture memory (534) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(535) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (535), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (534).

The controller (550) may manage coding operations of the source coder(530), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (545). The entropy coder (545)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (540) may buffer the coded video sequence(s) as createdby the entropy coder (545) to prepare for transmission via acommunication channel (560), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(540) may merge coded video data from the video coder (503) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (550) may manage operation of the video encoder (503).During coding, the controller (550) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (503) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (503) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (540) may transmit additional datawith the encoded video. The source coder (530) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bepredicted by a combination of the first reference block and the secondreference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 6 shows a diagram of a video encoder (603) according to anotherembodiment of the disclosure. The video encoder (603) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (603) is used in theplace of the video encoder (303) in the FIG. 3 example.

In an HEVC example, the video encoder (603) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (603) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (603) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(603) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (603) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 6 example, the video encoder (603) includes the interencoder (630), an intra encoder (622), a residue calculator (623), aswitch (626), a residue encoder (624), a general controller (621), andan entropy encoder (625) coupled together as shown in FIG. 6.

The inter encoder (630) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (622) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (622) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (621) is configured to determine general controldata and control other components of the video encoder (603) based onthe general control data. In an example, the general controller (621)determines the mode of the block, and provides a control signal to theswitch (626) based on the mode. For example, when the mode is the intramode, the general controller (621) controls the switch (626) to selectthe intra mode result for use by the residue calculator (623), andcontrols the entropy encoder (625) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(621) controls the switch (626) to select the inter prediction resultfor use by the residue calculator (623), and controls the entropyencoder (625) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (623) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (622) or the inter encoder (630). Theresidue encoder (624) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (624) is configured to convert the residuedata in the frequency domain, and generate the transform coefficients.The transform coefficients are then subject to quantization processingto obtain quantized transform coefficients. In various embodiments, thevideo encoder (603) also includes a residue decoder (628). The residuedecoder (628) is configured to perform inverse-transform, and generatethe decoded residue data. The decoded residue data can be suitably usedby the intra encoder (622) and the inter encoder (630). For example, theinter encoder (630) can generate decoded blocks based on the decodedresidue data and inter prediction information, and the intra encoder(622) can generate decoded blocks based on the decoded residue data andthe intra prediction information. The decoded blocks are suitablyprocessed to generate decoded pictures and the decoded pictures can bebuffered in a memory circuit (not shown) and used as reference picturesin some examples.

The entropy encoder (625) is configured to format the bitstream toinclude the encoded block. The entropy encoder (625) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (625) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 7 shows a diagram of a video decoder (710) according to anotherembodiment of the disclosure. The video decoder (710) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (710) is used in the place of the videodecoder (310) in the FIG. 3 example.

In the FIG. 7 example, the video decoder (710) includes an entropydecoder (771), an inter decoder (780), a residue decoder (773), areconstruction module (774), and an intra decoder (772) coupled togetheras shown in FIG. 7.

The entropy decoder (771) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (772) or the inter decoder (780), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (780); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (772). The residual information can be subject to inversequantization and is provided to the residue decoder (773).

The inter decoder (780) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (772) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (773) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (773) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (771) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (774) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (773) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (303), (503), and (603), and thevideo decoders (310), (410), and (710) can be implemented using anysuitable technique. In an embodiment, the video encoders (303), (503),and (603), and the video decoders (310), (410), and (710) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (303), (503), and (503), and the videodecoders (310), (410), and (710) can be implemented using one or moreprocessors that execute software instructions.

Aspects of the disclosure provide techniques to simplify affine motioncompensation with prediction offsets.

Generally, a motion vector for a block can be coded either in anexplicit way, to signal the difference to a motion vector predictor(e.g., advanced motion vector prediction or AMVP mode); or in animplicit way, to be indicated completely from one previously coded orgenerated motion vector. The later one is referred to as merge mode,meaning the current block is merged into a previously coded block byusing its motion information.

Both the AMVP mode and the merge mode construct candidate list duringdecoding.

FIG. 8 shows an example of spatial and temporal candidates in someexamples.

For the merge mode in the inter prediction, merge candidates in acandidate list are primarily formed by checking motion information fromeither spatial or temporal neighboring blocks of the current block. Inthe FIG. 8 example, candidate blocks A1, B1, B0, A0 and B2 aresequentially checked. When any of the candidate blocks are validcandidates, for example, are coded with motion vectors, then, the motioninformation of the valid candidate blocks can be added into the mergecandidate list. Some pruning operation is performed to make sureduplicated candidates will not be put into the list again. The candidateblocks A1, B1, B0, A0 and B2 are adjacent to corners of the currentblock, and are referred to as corner candidates.

After spatial candidates, temporal candidates are also checked into thelist. In some examples, the current block's co-located block in aspecified reference picture is found. The motion information at C1position (bottom right corner of the current block) of the co-locatedblock will be used as temporal merge candidate. If the block at thisposition is not coded in inter mode or not available, C1 position (atthe outer bottom right corner of the center of the co-located block)will be used instead. The present disclosure provide techniques tofurther improve merge mode.

The advanced motion vector prediction (AMVP) mode in HEVC refers tousing spatial and temporal neighboring blocks' motion information topredict the motion information of the current block, while theprediction residue is further coded. Examples of spatial and temporalneighboring candidates are shown in FIG. 8 as well.

In some embodiments, in AMVP mode, a two-candidate motion vectorpredictor list is formed. For example, the list includes a firstcandidate predictor and a second candidate predictor. The firstcandidate predictor is from the first available motion vector from theleft edge, in the order of spatial A0, A1 positions. The secondcandidate predictor is from the first available motion vector from thetop edge, in the order of spatial B0, B1 and B2 positions. If no validmotion vector can be found from the checked locations for either theleft edge or the top edge, no candidate will be filled in the list. Ifthe two candidates available and are the same, only one will be kept inthe list. If the list is not full (with two different candidates), thetemporal co-located motion vector (after scaling) from C0 location willbe used as another candidate. If motion information at C0 location isnot available, location C1 will be used instead.

In some examples, if there are still no enough motion vector predictorcandidates, zero motion vector will be used to fill up the list.

In some embodiments, prediction offsets can be signaled on top ofexisting merge candidates. For example, a technique that is referred toas ultimate motion vector expression (UMVE) uses a special merge mode inwhich an offset (both magnitude and direction) on top of the existingmerge candidates is signaled. In this technique, a few syntax elements,such as a prediction direction IDX, a base candidate IDX, a distanceIDX, a search direction IDX, and the like, are signaled to describe suchan offset. For example, the prediction direction IDX is used to indicatewhich of the prediction directions (temporal prediction direction, e.g.,L0 reference direction, L1 reference direction or L0 and L1 referencedirections) is used for UMVE mode. The base candidate IDX is used toindicate which of the existing merge candidates is used as the startpoint (based candidate) to apply the offset. The distance IDX is used toindicate how large the offset is from the starting point (along x or ydirection, but not both). The offset magnitude is chosen from a fixnumber of selections. The search direction IDX is used to indicate thedirection (x or y, +or − direction) to apply the offset.

In an example, assuming the starting point MV is MV_S, the offset isMV_offset. Then the final MV predictor will be MV_final=M_S+MV_offset.

FIG. 9 shows examples for UMVE according to an embodiment of thedisclosure. In an example, the starting point MV is shown by (911) (forexample according to the prediction direction IDX and base candidateIDX), the offset is shown by (912) (for example according to thedistance IDX and the search direction IDX), and the final MV predictoris shown by (913) in FIG. 9. In another example, the starting point MVis shown by (921) (for example according to the prediction direction IDXand base candidate IDX), the offset is shown by (922) (for exampleaccording to the distance IDX and the search direction IDX), and thefinal MV predictor is shown by 923 in FIG. 9.

FIG. 10 shows examples for UMVE according to an embodiment of thedisclosure. For example, the starting point MV is shown by (1011) (forexample according to the prediction direction IDX and base candidateIDX). In the FIG. 10 example, 4 search directions, such as +Y, −Y, +Xand −X, are used, and the four search directions can be indexed by 0, 1,2, 3. The distance can be indexed by 0 (0 distance to the starting pointMV), 1 (1 s to the starting point MV), 2 (2 s to the starting point MV),3 (3 s to the starting point), and the like. Thus, when the searchdirection IDX is 3, and the distance IDX is 2, the final MV predictor isshown as 1015.

In another example, the search direction and the distance can becombined for indexing. For example, the starting point MV is shown by(1021) (for example according to the prediction direction IDX and basecandidate IDX). The search direction and the distance are combined to beindexed by 0-12 as shown in FIG. 10.

According to an aspect of the disclosure, affine motion compensation, bydescribing a 6-parameter (or a simplified 4-parameter) affine model fora coding block, can efficiently predict the motion information forsamples within the current block. More specifically, in an affine codedor described coding block, different part of the samples can havedifferent motion vectors. The basic unit to have a motion vector in anaffine coded or described block is referred to as a sub-block. The sizeof a sub-block can be as small as 1 sample only; and can be as large asthe size of current block.

When an affine mode is determined, for each sample in the current block,its motion vector (relative to the targeted reference picture) can bederived using such a model (e.g., 6 parameter affine motion model or 4parameter affine motion model). In order to reduce implementationcomplexity, affine motion compensation is performed on a sub-blockbasis, instead of on a sample basis. That means, each sub-block willderive its motion vector and for samples in each sub-block, the motionvector is the same. A specific location of each sub-block is assumed,such as the top-left or the center point of the sub-block, to be therepresentative location. In one example, such a sub-block size contains4×4 samples.

In general, an affine motion model has 6 parameters to describe themotion information of a block. After the affine transformation, arectangular block will become a parallelogram. In an example, the 6parameters of an affine coded block can be represented by 3 motionvectors at three different locations of the block.

FIG. 11 shows an example of a block (1100) with an affine motion model.The block (1100) uses motion vectors {right arrow over (v₀)}, {rightarrow over (v₁)}, and {right arrow over (v₂)} at three corner locationsA, B and C to describe the motion information of the affine motion modelused for the block (1100). These locations A, B and C are referred to ascontrol points.

In simplified example, an affine motion model uses 4 parameters todescribe the motion information of a block based on an assumption thatafter the affine transformation, the shape of the block does not change.Therefore, a rectangular block will remain a rectangular and same aspectratio (e.g., height/width) after the transformation. The affine motionmodel of such a block can be represented by two motion vectors at twodifferent locations, such as at corner locations A and B.

FIG. 12 shows examples of affine transformation for a 6-parameter affinemode (using 6-parameter affine model) and a 4-parameter affine mode(using 4-parameter affine model).

In an example, when assumptions are made such that the object only haszooming and translational motions, or the object only has rotation andtranslation models, then the affine motion model can be furthersimplified to a 3-parameter affine motion model with 2 parameters toindicate the translational part and 1 parameter to indicate either ascaling factor for zooming or an angular factor for rotation.

According to an aspect of the disclosure, when affine motioncompensation is used, two signaling techniques can be used. The twosignaling techniques are referred to as a merge mode based signalingtechnique and a residue (AMVP) mode based signaling technique.

For the merge mode based signaling technique, the affine information ofthe current block is predicted from previously affine coded blocks. Inone method, the current block is assumed to be in the same affine objectas the reference block, so that the MVs at the control points of thecurrent block can be derived from the reference block's model. The MVsat the current block' other locations are just linearly modified in thesame way as from one control point to another in the reference block.This method is referred to as model based affine prediction. In anothermethod, neighboring blocks' motion vectors are used directly as themotion vectors at current block's control points. Then motion vectors atthe rest of the block are generated using the information from thecontrol points. This method is referred as control point based affineprediction. In either method, no residue components of the MVs atcurrent block are to be signaled. In other words, the residue componentsof the MVs are assumed to be zero.

For the residue (AMVP) mode based signaling technique, affineparameters, or the MVs at the control points of the current block, areto be predicted. Because there are more than one motion vectors to bepredicted, the candidate list for motion vectors at all control pointsis organized in grouped way such that each candidate in the listincludes a set of motion vector predictors for all control points. Forexample, candidate 1={predictor for control point A, predictor forcontrol point B, predictor for control point C}; candidate 2={predictorfor control point A, predictor for control point B, predictor forcontrol point C}, etc. The predictor for the same control point indifferent candidates can be the same or different. The motion vectorpredictor flag ((mvp 10 flag for List 0 or mvp 11 flag for List 1) willbe used to indicate which candidate from the list is chosen. Afterprediction, the residue part of the parameter, or the differences of theactual MVs to the MV predictors at the control points, are to besignaled. The MV predictor at each control point can also come frommodel based affine prediction from one of its neighbors, using themethod described from the above description for merge mode based osignaling technique.

In some related methods, affine parameters for a block can be eitherpurely derived from neighboring block's affine model or control points'MV predictor, or from explicitly signal the MV differences at thecontrol points. However, in many cases the non-translational part of theaffine parameters is very close to zero. Using unrestricted MVdifference coding to signal the affine parameters has redundancy.

Aspects of the disclosure provide new techniques to better represent theaffine motion parameters therefore improve coding efficiency of affinemotion compensation. More specifically, to predict affine modelparameters in a more efficient way, the translational parameters of ablock are represented using a motion vector prediction, in the same wayor similar way as that is for a regular inter prediction coded block.For example, the translational parameters can be indicated from a mergecandidate. For the non-translational part, a few typically usedparameters such as rotation parameter and zooming parameter arepre-determined with a set of fixed offset values. These values areconsidered as some refinements or offset around the default value. Theencoder can evaluate the best option from these values and signal theindex of the choice to the decoder. The decoder then restores the affinemodel parameters using 1) the decoded translational motion vector and 2)the index of the chosen non-translational parameters.

In the following description, 4-parameter affine model is used as anexample, the methods described in the following description can beextended to other motion models, or affine models with different numbersof parameters as well, such as 6-parameter affine model, and the like.In some of the following description, the model used may not be alwaysaffine model, but possibly other types of motion model.

In an example, a 4-parameter affine model is described, such as shown byEq. 1

$\begin{matrix}\left\{ \begin{matrix}{x^{\prime} = {{\rho \; \cos \mspace{11mu} {\theta \cdot x}} + {\rho \; \sin \mspace{11mu} {\theta \cdot y}} + c}} \\{y^{\prime} = {{{- \rho}\; \sin \mspace{11mu} {\theta \cdot x}} + {\rho \; \cos \mspace{11mu} {\theta \cdot y}} + f}}\end{matrix} \right. & {{Eq}.\mspace{14mu} 1}\end{matrix}$

where ρ is the scaling factor for zooming, θ is the angular factor forrotation, and (c, f) is the motion vector to describe the translationalmotion. (x, y) is a pixel location in the current picture, (x′, y′) is acorresponding pixel location in the reference picture.

In an embodiment, a motion vector predictor (MVP) is used to representthe translational part of the affine model. For example, MVP=(c, f). TheMVP can be derived from one of the existing regular (non-subblock based)merge candidates. In an example, the MVP can also be signaled in AMVPmode. If a merge candidate is bi-predicted, one of the MVs in a pair forthis candidate may be used as the MVP for translational parametersprediction.

For non-translational parts of the affine model, when ρ=1 and θ=0, theaffine model goes back to the translational motion model. ρ=1 isconsidered as a default value for the scaling factor, and θ=0 isconsidered as a default value for the angular factor.

According to an aspect of the disclosure, a mapping of indexes andoffset values (or delta values) around a default value is pre-definedand known at the encoder side and the decoder side. Thus, at the encoderside, the encoder can determine the offset value, and then signal anindex corresponding to the offset value according to the mapping. At thedecoder side, the decoder can decode the index from the coded videostream, and determine the corresponding offset value to the decodedindex according to the mapping. Further, the decoder can determine anaffine model parameter based on the default value for the affine modelparameter and the determined offset value.

It is noted that, in some embodiments, the mapping of the indexes andoffset values of an affine parameter can be represented in the form of amapping of the indexes and the values of the affine parameter. Further,in some embodiments, the mapping of the indexes and the offset values ofan affine parameter can be represented in the form of a mapping of theindexes and calculated function values of the affine parameter.

In some examples, a set of pre-defined delta values around the defaultvalues of the affine model parameters (e.g., ρ=1 and θ=0) are used toapproximate the actual affine model used in the current block. Becausethe number of delta values is limited, this technique can be regarded asa quantized version of signaling affine parameters. Below are a fewexamples to specify the values of ρ and θ. In the examples, idx_ρdenotes the index for the offset value to the default value of ρ (e.g.,ρ=1), and idx_θ denotes to the index for the offset value to the defaultvalue of θ (e.g., θ=0). When idx_ρ and idx_θ are 0, the affine model isa translational model. When idx_ρ and idx_θ are not zero, then smallvariations from the default values are used in affine model parametersprediction.

In an example, n is a preset and signaled for determining scalingparameter. The mapping of the idx_ρ and ρ can be defined according toTable 1:

TABLE 1 Mapping of the idx_ρ and ρ Idx_ρ 0 1 2 3 4 5 6 7 8 ρ 1 1 + n 1 −n 1 + 2n 1 − 2n 1 + 4n 1 − 4n 1 + 8n 1 − nn

The signaling of n can be done at block level, CTU level, slice/picturelevel or sequence level. For example, n can be 1/16.

Table 2 shows another mapping example of the idx_ρ and ρ:

TABLE 2 Mapping of the idx_ρ and ρ Idx_ρ 0 1 2 3 4 5 6 7 8 ρ 1 1 + n 1 −n 1 + 2n 1 − 2n 1 + 3n 1 − 3n 1 + 4n 1 − 4n

In another example, the mappings of idx_θ to sin θ and (cos θ)̂2 aredefined according to Table 3:

TABLE 3 Mapping of idx_θ to sin θ and (cos θ){circumflex over ( )}2Idx_θ 0 1 2 3 4 5 6 7 8 (cosθ){circumflex over ( )}2 1 1 1/32 1 1/32 11/16 1 1/16 1⅛ 1⅛ 1¼ 1¼ sinθ 0 Sqrt( 1/32) −Sqrt( 1/32) ¼ −¼ Sqrt(⅛)−Sqrt(⅛) ½ −½

In another example, α is a preset and signaled for determining angleparameter. The mapping of idx_θ and θ can be defined according to Table4:

TABLE 4 Mapping of the idx_θ and θ Idx_θ 0 1 2 3 4 5 6 7 8 θ 1 +α −α +2α−2α +3α −3α +4α −4α

The signaling of α can be done at block level, CTU level, slice/picturelevel or sequence level.

Table 5 shows another mapping example of idx_θ and θ:

TABLE 5 Mapping of the idx_θ and θ Idx_θ 0 1 2 3 4 5 6 7 8 θ 1 +α −α +2α−2α +4α −4α +8α −8α

In the above examples, the binarization of the indices can be configuredin the following way: 1 bit is used to signal if the index is 0 or not.If yes, not additional bit is needed. If not, in one embodiment,variable length coding, such as truncated binary, exponential-golombcode, etc, applies to index from 1˜8. In another embodiment, if not, fixlength coding is used to signal index from 1-8.

It is noted that the possible number of delta values is not limited to8. Other suitable values, such as 4, 16, etc. can be used.

In some embodiments, the affine model is predicted using control pointMVs at 2 or 3 corners, either by the model-based prediction or cornercontrol points based prediction. After the motion vector prediction fortwo or three control points, the MV differences of these control pointsare determined and signaled at the encoder side. Then, the decoder sidecan decode the MV difference, and perform motion vector prediction todetermine MVs at the control points.

Similarly to the above mapping examples, a set of pre-defined deltavalues are used to represent the MV difference. In one embodiment, ausage flag is signaled first, to indicate whether the MVD (motion vectordifference) is zero or is signaled according to the present disclosure.In an example, when the MVD is signaled according to the presentdisclosure, MV difference is assumed to be x direction or y direction,but not both. In this case, for each MVD, a combination of direction anddistance indices can be used to represent this MVD. Table 6 shows amapping example of direction indexes (direction IDXs) to directions, andTable 7 shows a mapping example of distance indexes (distance IDXs) todistances in term of pixels. The direction index represents thedirection of the MVD relative to the starting point (MV predictor).

TABLE 6 Mapping of direction IDXs to directions Direction IDX 00 01 1011 x-axis + − N/A N/A y-axis N/A N/A + −

TABLE 7 Mapping of distance IDXs to distances Distance IDX 0 1 2 3 4 5 67 Pixel ¼-pel ½-pel 1-pel 2-pel 4-pel 8-pel 16-pel 32-pel distance

In another example, the directions of MV difference include x directiononly, y direction only, and diagonal directions (e.g., 45°, 135°, 225°,and 315°). Table 8 shows a mapping example of direction indexes(direction IDXs) to directions.

TABLE 8 Mapping of direction IDXs to directions Direction IDX 000 001010 011 100 101 110 111 x-axis + + 0 − − − 0 + y-axis 0 + − − 0 + + −

In Table 8, for each of the 8 directions, the value determined from thedistance index will be applied to each of the x and y direction that isnon-zero. For example, when the distance index is 2, and the directionindex is 3 (011 in binary), 1-pel offset is applied to the −x direction,and 1-pel offset is applied to the −y direction from the starting pointMV. In another example, when the distance index is 2, and the directionindex is 2 (010 in binary), 1-pel offset is applied to −y direction fromthe starting point MV.

In an embodiment, the above approximation is done for every MVdifference of the affine mode.

In another embodiment, after coding of MV difference for the firstcontrol point, the MV difference for the first control point is used topredict other MV difference(s) for other control points beforeperforming MVD coding for other MVD(s). This is referred as MVDprediction. After MVD prediction for a second control point for example,the MVD prediction error of the second control point is to be codedusing the methods proposed in this disclosure, that is, use apre-defined set of values to approximate the actual value.

FIG. 13 shows a flow chart outlining a process (1300) according to anembodiment of the disclosure. The process (1300) can be used in thereconstruction of a block coded in intra mode, so to generate aprediction block for the block under reconstruction. In variousembodiments, the process (1300) are executed by processing circuitry,such as the processing circuitry in the terminal devices (210), (220),(230) and (240), the processing circuitry that performs functions of thevideo encoder (303), the processing circuitry that performs functions ofthe video decoder (310), the processing circuitry that performsfunctions of the video decoder (410), the processing circuitry thatperforms functions of the intra prediction module (452), the processingcircuitry that performs functions of the video encoder (503), theprocessing circuitry that performs functions of the predictor (535), theprocessing circuitry that performs functions of the intra encoder (622),the processing circuitry that performs functions of the intra decoder(772), and the like. In some embodiments, the process (1300) isimplemented in software instructions, thus when the processing circuitryexecutes the software instructions, the processing circuitry performsthe process (1300). The process starts at (S1301) and proceeds to(S1310).

At (S1310), prediction information of a block is decoded from a codedvideo bitstream. The prediction information is indicative of an affinemode that uses an affine model of motion information, such as6-parameter affine motion model, 4-parameter affine motion model and thelike. The prediction information includes an index for a predictionoffset (to a default value) associated with the affine model.

At (S1320), parameters of the affine model are determined based on theindex and a pre-defined mapping of indexes and offset values. Tables 1-8show various examples of pre-defined mappings of indexes and offsetvalues, and can be used to determine parameters of the affine model.

At (S1330), samples of the block are reconstructed according to theaffine model. In an example, a reference pixel in the reference picturethat corresponds to a pixel in the block is determined according to theaffine model. Further, the pixel in the block is reconstructed accordingto the reference pixel in the reference picture. Then, the processproceeds to (S1399) and terminates.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 14 shows a computersystem (1400) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 14 for computer system (1400) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1400).

Computer system (1400) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1401), mouse (1402), trackpad (1403), touchscreen (1410), data-glove (not shown), joystick (1405), microphone(1406), scanner (1407), camera (1408).

Computer system (1400) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1410), data-glove (not shown), or joystick (1405), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1409), headphones(not depicted)), visual output devices (such as screens (1410) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1400) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1420) with CD/DVD or the like media (1421), thumb-drive (1422),removable hard drive or solid state drive (1423), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1400) can also include an interface to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters that attached to certain general purpose dataports or peripheral buses (1449) (such as, for example USB ports of thecomputer system (1400)); others are commonly integrated into the core ofthe computer system (1400) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1400) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1440) of thecomputer system (1400).

The core (1440) can include one or more Central Processing Units (CPU)(1441), Graphics Processing Units (GPU) (1442), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1443), hardware accelerators for certain tasks (1444), and so forth.These devices, along with Read-only memory (ROM) (1445), Random-accessmemory (1446), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (1447), may be connectedthrough a system bus (1448). In some computer systems, the system bus(1448) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (1448),or through a peripheral bus (1449). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (1441), GPUs (1442), FPGAs (1443), and accelerators (1444) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1445) or RAM (1446). Transitional data can be also be stored in RAM(1446), whereas permanent data can be stored for example, in theinternal mass storage (1447). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1441), GPU (1442), massstorage (1447), ROM (1445), RAM (1446), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1400), and specifically the core (1440) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1440) that are of non-transitorynature, such as core-internal mass storage (1447) or ROM (1445). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1440). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1440) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1446) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1444)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

Appendix A: Acronyms

-   JEM: joint exploration model-   VVC: versatile video coding-   BMS: benchmark set-   MV: Motion Vector-   HEVC: High Efficiency Video Coding-   SEI: Supplementary Enhancement Information-   VUI: Video Usability Information-   GOPs: Groups of Pictures-   TUs: Transform Units,-   PUs: Prediction Units-   CTUs: Coding Tree Units-   CTBs: Coding Tree Blocks-   PBs: Prediction Blocks-   HRD: Hypothetical Reference Decoder-   SNR: Signal Noise Ratio-   CPUs: Central Processing Units-   GPUs: Graphics Processing Units-   CRT: Cathode Ray Tube-   LCD: Liquid-Crystal Display-   OLED: Organic Light-Emitting Diode-   CD: Compact Disc-   DVD: Digital Video Disc-   ROM: Read-Only Memory-   RAM: Random Access Memory-   ASIC: Application-Specific Integrated Circuit-   PLD: Programmable Logic Device-   LAN: Local Area Network-   GSM: Global System for Mobile communications-   LTE: Long-Term Evolution-   CANBus: Controller Area Network Bus-   USB: Universal Serial Bus-   PCI: Peripheral Component Interconnect-   FPGA: Field Programmable Gate Areas-   SSD: solid-state drive-   IC: Integrated Circuit-   CU: Coding Unit

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof

1. A method for video decoding in a decoder, comprising: decodingprediction information of a block in a current picture from a codedvideo bitstream, the prediction information including a first index of aplurality of indexes for prediction offset associated with an affinemodel in an inter prediction mode, the affine model being used totransform between the block and a reference block in a reference picturethat has been reconstructed; changing a first parameter of the affinemodel according to which offset value of a plurality of offset values isdetermined to be associated with the first index, the plurality ofoffset values being mapped to the plurality of indexes; andreconstructing at least a sample of the block according to the affinemodel.
 2. The method of claim 1, further comprising: determining valuesof translational parameters of the affine model according to a motionvector; and changing a non-translational parameter for the affine modelaccording to the first index.
 3. The method of claim 2, wherein thefirst parameter of the affine model is a scaling factor in the affinemodel, and the offset value indicates a delta value to a default of thescaling factor in the affine model.
 4. The method of claim 2, whereinthe first parameter of the affine model is a rotation angle in theaffine model, and the offset value indicates a delta value to a defaultof the rotation angle in the affine model.
 5. The method of claim 1,wherein the first parameter of the affine model is a motion vectordifference; and the method further includes deriving the affine modelbased on a predicted motion vector and the motion vector difference. 6.The method of claim 5, wherein the offset value indicates a pixeldistance of the motion vector difference, and the method furtherincludes determining a direction of the motion vector difference basedon an index of another plurality of indexes.
 7. The method of claim 5,wherein the offset value indicates a pixel distance of the motion vectordifference, and the first index is associated with both a direction andthe pixel distance of the motion vector difference.
 8. The method ofclaim 5, further comprising: changing a second parameter of the affinemodel according to which of the plurality of offset values is associatedwith a second index of the plurality of indexes, wherein the first andsecond parameters are two motion vector differences for two controlpoints; determining motion vectors for the two control points based onthe two motion vector differences; and deriving a 4-parameter affinemodel based on the motion vectors of the two control points.
 9. Themethod of claim 5, further comprising: changing a second parameter ofthe affine model according to which of the plurality of offset values isassociated with a second index of the plurality of indexes, determininga third parameter of the affine model based on which of the plurality ofoffset values is associated with a third index of the plurality ofindexes, wherein the first, second, and third parameters are motionvector differences for three control points; determining motion vectorsfor the three control points based on the three motion vectordifferences; and deriving a 6-parameter affine model based on the motionvectors of the three control points.
 10. The method of claim 1, furthercomprising: changing a first motion vector difference for a firstcontrol point according to the offset value associated with the firstindex; predicting a second motion vector difference for a second controlpoint based on the first motion vector difference; decoding a predictionerror to correct the second motion vector difference from the codedvideo bitstream; determining a first motion vector for the first controlpoint and a second motion vector for the second control point based onthe first motion vector difference and the corrected second motionvector difference; and deriving the affine model at least based on thefirst motion vector for the first control point and the second motionvector for the second control point.
 11. An apparatus for videodecoding, comprising: processing circuitry configured to: decodeprediction information of a block in a current picture from a codedvideo bitstream, the prediction information including a first index of aplurality of indexes for prediction offset associated with an affinemodel in an inter prediction mode, the affine model being used totransform between the block and a reference block in a reference picturethat has been reconstructed; change a first parameter of the affinemodel according to which offset value of a plurality of offset values isdetermined to be associated with the first index, the plurality ofoffset values being mapped to the plurality of indexes; and reconstructat least a sample of the block according to the affine model.
 12. Theapparatus of claim 11, wherein the processing circuitry is configuredto: determine values of translational parameters of the affine modelaccording to a motion vector; and change a non-translational parameterfor the affine model according to the first index.
 13. The apparatus ofclaim 12, wherein the first parameter of the affine model is a scalingfactor in the affine model, and the offset value indicates a delta valueto a default of the scaling factor in the affine model.
 14. Theapparatus of claim 12, wherein the first parameter of the affine modelis a rotation angle in the affine model, and the offset value indicatesa delta value to a default of the rotation angle in the affine model.15. The apparatus of claim 11, wherein the first parameter of the affinemodel is a motion vector difference; and the processing circuitry isconfigured to derive the affine model based on a predicted motion vectorand the motion vector difference.
 16. The apparatus of claim 15, whereinthe offset value indicates a pixel distance of the motion vectordifference, and the processing circuitry is configured to determine adirection of the motion vector difference based on an index of anotherplurality of indexes.
 17. The apparatus of claim 15, wherein the offsetvalue indicates a pixel distance of the motion vector difference, andthe index is associated with both a direction and the pixel distance ofthe motion vector difference.
 18. The apparatus of claim 15, wherein theprocessing circuitry is configured to: change a second parameter of theaffine model according to which of the plurality of offset values isassociated with a second index of the plurality of indexes, the firstand second parameters being two motion vector differences for twocontrol points; determine motion vectors for the two control pointsbased on the two motion vector differences; and derive a 4-parameteraffine model based on the motion vectors of the two control points. 19.The apparatus of claim 15, wherein the processing circuitry isconfigured to change a second parameter of the affine model according towhich of the plurality of offset values is associated with a secondindex of the plurality of indexes, and determine a third parameter ofthe affine model based on which of the plurality of offset values isassociated with a third index of the plurality of indexes; the first,second, and third parameters are three motion vector differences forthree control points; determine motion vectors for the three controlpoints based on the three motion vector differences; and derive a6-parameter affine model based on the motion vectors of the threecontrol points.
 20. A non-transitory computer-readable medium storinginstructions which when executed by a computer for video decoding causethe computer to perform: decoding prediction information of a block in acurrent picture from a coded video bitstream, the prediction informationincluding a first index of a plurality of indexes for prediction offsetassociated with an affine model in an inter prediction mode, the affinemodel being used to transform between the block and a reference block ina reference picture that has been reconstructed; changing a firstparameter of the affine model according to which offset value of aplurality of offset values is determined to be associated with the firstindex, the plurality of offset values being mapped to the plurality ofindexes; and reconstructing at least a sample of the block according tothe affine model.